1. Field of the Invention
The present invention relates to a knock suppression control apparatus for an internal combustion engine, which apparatus is so arranged as to detect occurrence of knocking or knock event in the engine on the basis of level change of an ion current which flows by way of a spark plug upon combustion of an air-fuel mixture within a cylinder of the engine to thereby correct an engine control quantity so that occurrence of such knock event can be suppressed. More particularly, the present invention is concerned with a knock suppression control apparatus for an internal combustion engine, which apparatus is designed to avoid erroneous detection of the knock event notwithstanding of change or variation of the ion current which may be brought about due to retrofitting of the spark plugs and/or compositional difference of the air-fuel mixture charged in the engine cylinders and/or sooting state of the spark plug, to thereby ensure the knock suppression control with high reliability.
2. Description of Related Art
Heretofore, in the knock suppression control apparatus for the internal combustion engine, the control quantity or quantities for the engine have been so corrected as to suppress knock occurrence (e.g. by retarding the ignition timing, a typical one of engine control quantities) upon detection of the knock event in an effort to protect the engine against damage or injury due to the occurrence of knock event.
Further, the knock suppression control apparatus for the internal combustion engine in which the ion current flowing across the electrodes of the ignition plug is utilized for detecting the knock event can certainly detect occurrence of the knock on a cylinder-by-cylinder basis without resorting to any particular sensor dedicated for the knock detection, which is of course advantageous from the standpoint of cost reduction. For this reason, there have heretofore been proposed various types of knock suppression control apparatuses which can operate on the basis of the ion current.
In general, in the internal combustion engine, an air-fuel mixture charged into a combustion chamber defined within each of the engine cylinders is compressed by a piston moving reciprocatively within the cylinder, and a high voltage is applied to the spark plug disposed within the cylinder and exposed to the combustion chamber, whereby a spark is generated between the electrodes of the spark plug due to electric discharge. Thus,combustion of the compressed air-fuel mixture is triggered. Explosion energy resulting from the combustion is then converted into every for motion of the piston in the direction reverse to that in the compression stroke, which motion is translated into an output torque of the engine taken out therefrom via a crank shaft.
Upon combustion of the compressed air-fuel mixture within the combustion chamber in the engine cylinder, molecules prevailing within the combustion chamber are ionized. Thus, when a high voltage is applied to an ion current detecting electrode which is constituted by one of the electrodes of the spark plug, migration of ions carrying electric charges takes place between both the electrodes of the spark plug, which gives rise to flow of the ion current.
As is known in the art, magnitude of the ion current varies with a high sensitively in dependence on variation of the pressure prevailing within the combustion chamber and thus the ion current carries vibration components which are ascribable to the knock event. Thus, it is possible to decide on the basis of the ion current whether the knock event has occurred or not.
For having better understanding of the present invention, description will first be made of the technical background thereof in some detail. FIG. 3 is a circuit diagram showing generally and schematically a structure of a hitherto known or conventional knock suppression control apparatus for an internal combustion engine which is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 9108/1998 (JP-A-10-9108). In the apparatus shown in FIG. 3, a high voltage is applied distributively to spark plugs of individual engine cylinders, respectively, through the medium of a distributor 7.
The conventional apparatus shown in FIG. 3 is so designed as to extract vibration components ascribable to the knock event and superposed on an ion current i for counting knock pulses which are resulted from waveform shaping of the knock vibration components, to thereby make knock decision (i.e., decision as to occurrence or non-occurrence of the knock event) on the basis of the number of the counted pulses (hereinafter also referred to as the pulses number).
Referring to FIG. 3, there is provided in association with a crank shaft (not shown) of an internal combustion engine (not shown either, hereinafter also referred to simply as the engine) a crank angle sensor 1 designed to output a crank angle signal SGT which contains a number of pulses generated at a frequency depending on a rotation number or speed (rpm) of the engine.
The leading edges of the individual pulses contained in the crank angle signal SGT indicate reference positions for the individual engine cylinders (not shown) in terms of the crank angles, respectively. The crank angle signal SGT is supplied to an electronic control unit (ECU) 2 which includes by a microcomputer or the like to be used for performing various controls as well as arithmetic operations therefor.
More specifically, the electronic control unit (ECU) 2 further includes a counter 21 for counting the number of pulses (also referred to as the pulses number) N of a knock pulse train Kp inputted from a waveform processing means (described later on) and a CPU (central processing unit) 22 constituting a major part of the microprocessor for deciding occurrence or non-occurrence of the knock on the basis of the pulses number N.
The counter 21, the CPU 22 and the waveform processing means cooperate to serve as a knock detecting means.
The electronic control unit 2 is so designed or programmed as to fetch the engine operation information signals from various sensors (not shown) in addition to the crank angle signal SGT outputted from the crank angle sensor 1 and execute various arithmetic operations in dependence on the engine operation states, to thereby generate driving signals for a variety of actuators and devices inclusive of an ignition coil 4 and others.
An ignition signal P for driving the ignition coil 4 is applied to a base of a power transistor TR connected to a primary winding 4a of the ignition coil 4 for turning on/off the power transistor TR. More specifically, the power transistor TR is turned of f in response to the driving signal P, whereby a primary current i1 flowing through the primary winding of the ignition coil 4 is interrupted.
Upon interruption or breaking of the primary current i1, the primary voltage v1 making appearance across the primary winding 4a rises up steeply, as a result of which a secondary voltage v2 further boosted up is induced in a secondary winding 4b of the ignition coil 4 and makes appearance thereacross as a voltage of high level for ignition, which voltage is usually on the order of several ten kilovolts. Hereinafter, this voltage will also be referred to as the high ignition voltage or simply as the ignition voltage. In this manner, the ignition coil 4 generates the secondary voltage V2 (high ignition voltage) in conformance with the ignition timing of the engine.
The distributor 7 which is connected to an output terminal of the secondary winding 4b operates to apply distributively and apply the secondary voltage V2 sequentially to spark plugs 8a, . . . , 8d installed in the engine cylinders, respectively, in synchronism with the rotation of the engine, whereby spark discharges take place within the combustion chambers defined in the engine cylinders, respectively, triggering combustion or burning of the air-fuel mixture confined within the combustion chamber.
More specifically, with the spark discharges occurring across the spark plugs 8a, . . . , 8d in response to the application of the secondary voltage V2 in conformance with the ignition timing of the engine, the air-fuel mixture within the cylinders is fired or ignited.
Connected between one end of the primary winding 4a of the ignition coil 4 and the ground is a series circuit which is composed of a rectifier diode D1, a current limiting resistor R, a capacitor 9 connected in parallel with a Zener diode DZ serving for voltage limiting function, and a rectifier diode D2. The series circuit mentioned above constitutes a path for allowing a charging current to flow to the capacitor 9 which constitutes a bias voltage source for applying a bias voltage to one electrode of the spark plug for detecting the ion current.
More specifically, the capacitor 9 connected in parallel with the Zener diode DZ is electrically charged to a voltage level corresponding to a predetermined bias voltage VBi which is ordinarily on the order of several hundred volts by the charging current which flows under the primary voltage V1. Thus, the capacitor 9 serves as the bias voltage source for detecting the ion current i, as mentioned above. To this end, the capacitor 9 is so connected as to electrically discharge by way of the spark plug (8a, . . . , 8d) immediately after the ignition, allowing thus the ion current i to flow through the spark plug.
Connected to one end of the capacitor 9 are anodes of high-voltage diodes 11a, . . . , 11d, respectively, which have respective cathodes connected to one electrodes of the spark plugs 8a, . . . , 8d, respectively, with a same polarity as that of the firing or ignition voltage. On the other hand, connected to the other end of the capacitor 9 is a resistor 12 for detecting the ion current, which serves to convert the ion current i into a voltage signal and output it as an ion current detection voltage signal Ei.
The resistor 12 is connected to the other ends of the spark plugs 8a, . . . , 8d, respectively, via the ground and forms a path for the ion current i in cooperation with the capacitor 9 and the high-voltage diode 11a, . . . , 11d.
The ion current detection voltage signal Ei derived from the resistor 12 is shaped by a waveform shaper circuit 13 which thus outputs a waveform-shaped signal Fi. Subsequently, only the knock signal Ki is extracted from the waveform-shaped signal Fi through a band-pass filter 14. The knock signal Ki is then converted into a knock pulse train Kp through a comparison circuit 15 to be ultimately supplied to the counter 21 incorporated in the electronic control unit (ECU) 2.
The waveform shaper circuit 13, the band-pass filter 14 and the comparison circuit 15 cooperate to constitute a waveform processing means for extracting the knock pulse train Kp from the ion current detection voltage signal Ei.
The pulses number N of the knock pulses contained in the pulse train Kp is counted by the counter 21 of the electronic control unit 2 to be used for allowing the CPU (central processing unit) 22 to make decision as to whether the knock event occurs or not.
The pulses number N of the knock pulse train Kp bears a significant correlation with the intensity or magnitude of the knock. In other words, the pulses number N increases, as the magnitude of knock is larger.
Now, referring to FIG. 4 along with FIG. 3, operation of the conventional knock suppression control apparatus for the internal combustion engine will be described.
FIG. 4 is a timing chart for illustrating waveforms of the various signals mentioned above by reference to FIG. 3. It can be seen from FIG. 4 that the knock signal is superposed on the ion current i (see waveform-shaped signal Fi).
The electronic control unit 2 outputs the ignition signal P for turning on/off the power transistor TR on the basis of the crank angle signal SGT derived from the output signal of the crank angle sensor 1. The power transistor TR electrically conducts (i.e., assumes ON-state) when the ignition signal P is at a high or "H" level, to thereby allow the primary current i1 to flow through the primary winding 4a of the ignition coil 4, while interrupting the current i1 at the time point when the ignition signal P changes from the "H" level to a low or "L" level.
Upon interruption of the primary current i1, the primary voltage V1 rising steeply is induced in the primary winding 4a, as a result of which the capacitor 9 is charged with the current flowing along the charging path constituted by the rectifier diode D1, the current limiting resistor R, the capacitor 9 and the rectifier diode D2. Needless to say, charging of the capacitor 9 is terminated when the voltage appearing across the capacitor 9 has reached the level corresponding to the backward breakdown voltage of the Zener diode DZ, which voltage in turn corresponds to the bias voltage VBi.
In this manner, the capacitor 9, the Zener diode DZ and the diode D2 cooperate to constitute a bias means, wherein the capacitor 9 is charged under the effect of the high voltage making appearance at the low voltage side of the primary winding 4a upon interruption of the primary current i1.
When the primary voltage V1 is induced in the primary winding 4a as mentioned above, there is induced in the secondary winding 4b of the ignition coil 4 the secondary voltage V2 which is boosted up to a high ignition voltage on the order of several ten kilovolts. This secondary voltage V2 is applied distributively to the spark plugs 8a, . . . , 8d of the individual engine cylinders, respectively, by means of the distributor 7, which results in generation of the spark discharge at the plugs within the combustion chambers of the engine cylinders which are under control. Thus, the air-fuel mixture undergoes burning or combustion.
Upon combustion of the air-fuel mixture, ions are produced within the combustion chamber defined in the engine cylinder. The ion current i can thus flow under the bias voltage VBi charged in the capacitor 9. By way of example, let's assume that combustion of the air-fuel mixture takes place within the combustion chamber in which the spark plug 8a is disposed. Then, the ion current i flows along the current path extending from the capacitor 9 to the resistor 12 by way of the diode 11a and the spark plug 8a and then to the capacitor 9, as mentioned previously.
The ion current i is converted to the ion current detection voltage signal Ei by means of the resistor 12 (serving as the ion current detecting means), whereon the ion current detection voltage signal Ei is shaped to the signal Fi (FIG. 4) by means of the shaper circuit 13.
As can be seen in FIG. 4, the shaped signal Fi is of such waveform which allows the knock signal Ki to be easily extracted by clipping only the ion current component at a predetermined voltage level.
When a knock event occurs in the engine, signal components ascribable to the knocking vibrations are superposed on the ion current i. Thus, the shaped signal Fi assumes a waveform in which the knocking vibration components are superposed on the ion current.
The shaped signal Fi is supplied to the band-pass filter 14 and the comparison circuit 15 which constitute the waveform processing means.
Thus, the band-pass filter 14 selectively extracts only the knock signal Ki indicative of the knocking vibration frequency. On the other hand, the comparison circuit 15 compares the knock signal Ki with a predetermined reference level. As a result, the knock pulse train Kp is outputted from the comparison circuit 15 to be supplied to the counter 21 which is incorporated in the electronic control unit (ECU) 2.
The counter 21 of the electronic control unit 2 is designed to count the pulses number N of the knock pulse train Kp in response to a leading (rising or falling) edge thereof. The signal indicating the pulses number is then inputted to the CPU 22.
The pulses number N increases as the magnitude of the knock becomes larger. Thus, the CPU 22 of the electronic control unit 2 can decide or determine not only the occurrence or non-occurrence of the knock event but also the magnitude thereof on the basis of the pulses number N. By virtue of this feature, the control quantity (ignition timing) can be so corrected as to suppress the knock event on the basis of the pulses number N.
By way of example, when a count value of the pulses number N becomes equal to or greater than a predetermined number, occurrence of the knock event is decided. In that case, the ignition timing (the timing at which the primary current i1 is interrupted) is correctively retarded by a predetermined quantity. Subsequently, so long as the occurrence of the knock event is still decided in succession, the retard quantity is progressively incremented until no occurrence of the knock event is decided.
The predetermined pulses number which is used as the reference value for comparison for the knock decision, as described previously, can be set to a value within a range of e.g. 5 to 20, although it depends on the engine rotation number and the waveform shaping level set in the comparison circuit 15.
In this way, by determining the retard quantity for retarding correctively the ignition timing in dependence on the result of decision made by the CPU 22, the ignition timing for the cylinder in which the knock has occurred can be corrected optimally, whereby occurrence of the knock event can be suppressed effectively.
At this juncture, it should be mentioned that the fuel is often admixed with an additive containing easily ionizable materials or substances such as Na (sodium), K (potassium) and/or the like in an effort to increase the engine power (output torque of the engine) as is known in the art. In that case, not only the ions produced upon combustion of the air-fuel mixture but also the ions produced due to ionization of the admixed substances themselves are detected simultaneously.
As a consequence, the ion current detection signal Ei will assume an increased level when compared with the case where the ordinary fuel containing no ionizable substances such as mentioned above is used. Consequently, the amplitude of the high-frequency vibration component (knock signal Ki) derived through the band-pass filter 14 will naturally increase, as a result of which noise components contained in the knock signal Ki will increase substantially proportionally to the knock components.
Further, it should also be mentioned that the amplitudes of the ion current detection voltage signal Ei and the knock signal Ki increase or decrease when the geometrical or structural factors of the spark plug 8a, . . . , 8d such as the shape of the electrodes thereof, inter-electrode distance and the like change of course, variation (increase or decrease) of the ion current detection voltage signal Ei due to the structural change of the electrodes of the spark plug is small when compared with the case where the fuel admixed with the additive containing the ionizable substance mentioned above is used. Parenthetically, such structural or geometrical change of the spark plug may be explained by the exchange of the originally installed spark plug (hereinafter referred to as the standard spark plug only for convenience of the description) by a fresh one (hereinafter also referred to as the nonstandard spark plug), i.e., by retrofitting of the spark plug.
In any case, when the spark plug 8a, . . . , 8d is replaced by a commercially available nonstandard plug which differs from the originally installed standard one or when the fuel admixed with the additive containing easily ionizable substances such as mentioned above is used, the amplitude of the knock signal Ki will assume different level or value when compared with the case where the standard spark plugs are employed or where the fuel containing no ionizable substance such as Na, K or the like is used.
However, in the conventional knock suppression control apparatuses, no measures are taken for coping with such amplitude variations or changes of the knock signal Ki as mentioned above.
Additionally, it should be mentioned that when the spark plug 8a, . . . , 8d is in the sooting state in which an insulation resistance value of the plug gap through which the ion current flows is low, a leak current of the magnitude which is determined by the insulation resistance value and the bias voltage VBi can flow through the plug gap, as a result of which the ion current i which contains the leak current component will be detected. In other words, when the sooting state prevails at the spark plug, reliability of detection of the ion current i will become degraded more or less.
As will now be appreciated, when the fuel admixed with the additive containing the easily ionizable material(s) or substance(s) is used or when the standard spark plug is replaced by a nonstandard spark plug which differs from the former in respect to the geometrical configuration, great difficulty will be encountered in realizing in a satisfactory manner the knock detection and the knock suppression control with the conventional knock suppression control apparatus while maintaining the hardware/software structures thereof as they are, i.e., without changing or modifying the hardware/software structure of the knock suppression control apparatus.
As is apparent from the foregoing description, the conventional knock suppression control apparatus for the internal combustion engine can certainly perform the knock suppression control on the basis of the ion current i. However, the knock suppression control apparatus known heretofore suffers a problem that erroneous knock detection and hence erroneous knock suppression control will be incurred due to level variation of the ion current detection voltage signal Ei when the fuel admixed with the additive containing easily ionizable substance(s) or when the standard spark plug is replaced by a non-standard one or when the sooting state occurs in the spark gap of the spark plug.